1. Field of the Invention
This invention relates to a process for distilling a multicomponent hydrocarbon mixture to produce a plurality of distillate products and in which all the distillation processes are divised so as to save energy and the heat supplied is efficiently recovered.
2. Background of the Invention
Distillation processes in which a multicomponent hydrocarbon mixture is separated into a plurality of distillate products are usually performed in such a way that a plurality of distillation columns, each of which is provided with a reboiler and a condenser, is installed in a proper sequence and the respective products are successively separated. FIG. 1 illustrates one example of such a distillation process with a ternary component system. Since in this type of distillation process the products are repeatedly heated and cooled, the heat energy required for the process is considerably large.
Thus, the economy of this heat energy is of industrially great significance and heretofore, for the purpose of saving energy in these distillation processes a number of systems have been proposed. The following are well-known:
(1) Petlyuk's distillation system;
(2) a distillation system utilizing the multi-effect principle; and
(3) a countercurrent type heat exchange system.
These systems are effectively utilized to reduce the above-described heat energy supply, but when applied to various multicomponent hydrocarbon mixtures, it is found that they inherently have respective limits depending on the properties of the multicomponent hydrocarbon mixtures. Each of the systems will be explained below. Further descriptions of such systems may be found in texts such as "Separation Processes" by C. J. King, McGraw-Hill (1971).
(1) Petlyuk et al
proposed that the heat energy required for the distillation of a multicomponent hydrocarbon mixture can be reduced by changing the combination of distillation columns. For the sake of simplicity, an example of a ternary component system is shown in FIG. 2. In the figure the first column sloppy separates the multicomponent mixture while the second column sharply separates the components according to their specifications. The rate of reduction of the heat energy required for the distillation process by the system, however, has a limit, and moreover, is subject to fluctuations depending on the composition of the feed stock. Furthermore, since both the vapor phase stream and the liquid phase stream are mutually exchanged between the adjacent columns Petlyuk's distillation system is difficult to control. Novertheless, when combined with many other systems, this system can be utilized as a distillation system which has an improved controllability on the whole.
(2) A distillation system utilizing the multi-effect principle.
In the case where use is made in a certain distillation column A of a heat source whose temperature is sufficiently higher than the bottom temperature of the column A, the heat energy of the heat source is sometimes utilized to heat the reboiler of another distillation column B, and then the heat recovered from the condenser of distillation column B is utilized to heat the reboiler of distillation column A. A technique utilizing heat repeatedly in this way is called a multi-effect technique. When applying this technique to a ternary system, distillation processes such as shown in FIGS. 3 and 4 may be considered. In these figures distillation column B is operated under a pressure higher than that of distillation column A.
FIG. 3 illustrates an example using a binary component system, and FIG. 4 illustrates an example using a ternary component system. In the above described system, the rate of reduction of the heat required for the distillation process is determined by the balance between the condenser load in the high pressure distillation column and the reboiler load in the low pressure distillation column. For this reason, a drastic reduction of the required heat energy cannot be expected without the proper choice of the high pressure distillation column. Also, if there is too large a temperature difference between the reboiler and the condenser this system cannot be applied.
(3) Countercurrent type heat exchange system.
In this system, in order to recover the heat from the heat source streams to be heat-exchanged by the heat sink streams there are installed a plurality of heat exchanger groups, which are formed by grouping a number of heat exchangers. The heat sink streams are successively brought into contact with a plurality of heat exchangers in the heat exchanger groups that are at different temperature levels so as to recover the heat from the heat source streams. In this system of heat recovery the heat source and sink streams are brought in countercurrent contact throughout the whole heat exchange system, so that this heat exchange system is a thermodynamically less irreversible system under the given conditions.
However, this system is effective to some degree in reducing the heat required for the distillation process, but when applied simply to the above-described ordinary distillation process, it is found that heat recovery from a condenser at a low temperature level is very difficult. Hence, a drastic recovery of heat cannot be expected.
The present inventors, who thoroughly investigated the prior-known energy saving techniques, and as a result found that the mere application of these techniques to the ordinary distillation process can show only a limited effect, were able to establish a novel technique of distillation of a multicomponent system by the application of the most suitable combination of the multi-effect principle technique and the countercurrent type heat exchange system and by limiting the multicomponent system to which the process is applied as well as the combination of distillation columns.